“We have known that for decades leopard sharks (Triakis semifasciata) aggregate along the La Jolla’s open coast, but no one knew why they came and what made the site special,” said Scripps Institution of Oceanography PhD student Andy Nosal, who recently concluded a study of these sharks.

Nosal confirmed that although practically all local leopards are mature, pregnant females, they don’t gather to give birth because the pregnancies are early stage. Other congregations of pregnant female leopards are also found throughout California and Baja, Mexico, but they choose quiet estuaries instead of open coastline for their summer sojourn.

Certainly, the La Jolla site is less rugged than much of the coast because it is doubly bordered by beach on one side and cliffs on another. This arrangement reduces predatory opportunities by mature male sea lions. And because the site is within the confines of the San Diego-La Jolla Underwater Park Ecological Reserve, depletion — or worse — by humans is prevented.

Nosal looked closely at other physical aspects of the La Jolla Shores environment and discovered that most of the site’s unique features are due to the presence of the nearby submerged canyon. The canyon is actually the conclusion of Rose Canyon Fault, known by its terrestrial link as La Jolla Parkway. The submarine canyon, which acts to slow down incoming swells, is responsible for the Shores southwest corner having the gentlest surf of all San Diego beaches.

Little water movement, combined with sunny skies, generates warmer water in this corner pocket than that found in the surrounding water. Toasty, calm conditions mean pregnant sharks expend less energy staying warm and holding position in the shallows so they can instead invest energy into speeding embryo development. The canyon also supplies the sharks with food security, primarily market squid (Doryteuthis opalescens), which are hunted during nightly forays into the depths.

Thus, while the La Jolla site is open coastal, it provides assets not unlike those seen in estuaries. But there is more to the story because where the La Jolla Beach & Tennis Club stands today was, in the past, an estuarine lagoon. Evidence comes from driftwood, reeds and 7,000-year-old fossils (from a brackish water invertebrate) collected in the submarine canyon’s mud, silt and clay substrate.

The ancient lagoon’s only remnant is a small duck pond on the club’s property. While this does not imply that today’s sharks are somehow genetically programmed to return to the lost lagoon, benefits from the lagoon would have been icing on the cake. Whatever extra goodness the long-ago lagoon supplied the leopard sharks, having a no-take reserve in today’s environment has been key to preventing their local extinction from overfishing. Regarding commerce, the leopard shark’s continued presence has benefited La Jolla’s business community through leopard-shark ecotourism.

Nosal’s work has not only demystified some of what makes La Jolla’s leopard sharks tick, but it has demonstrated the value of small no-take reserves in protecting marine life that exhibits a singular behavior.

“This is a beautiful example of how vulnerable, pregnant sharks can be protected by a no-take reserve to benefit the sharks, the public, La Jolla’s economy and science,” Nosal said. “There is comfort knowing these sharks will be protected for generations to come.”

— Judith Lea Garfield, naturalist and underwater photographer, has authored two natural history books about the underwater park off La Jolla Cove and La Jolla Shores. Send comments to jgarfield@ucsd.edu.